1. Field of the Invention
The present invention relates to a charging device employed in an electrophotographic image forming apparatus such as a copying machine and a printer as well as an image forming apparatus provided with the charging device.
2. Description of the Related Art
In a conventional electrophotographic image forming apparatus such as a copying machine and a printer, a toner image formed on a photosensitive member, which is generally used as an electrostatic latent image carrier, is transferred onto a transfer member, and the transferred toner image is fixed on the transfer member. More specifically, a charging device uniformly charges the photosensitive member. The electrostatic latent image is formed on the charged photosensitive member by an image exposing device. A developing device develops the electrostatic latent image on the photosensitive member to form a toner image. A transfer device transfers the toner image onto the transfer member. After this transfer, toner remaining on the photosensitive member is removed from the photosensitive member by a cleaning device.
In recent years, there has been proposed a so-called cleanerless image forming apparatus from which a cleaning device is eliminated in order to comply with demand for reduction of size and cost of the image forming apparatus. For example, U.S. Pat. Nos. 5,148,219 and 5,221,946 have disclosed cleanerless image forming apparatuses, in which a developing device additionally has a cleaning function for removing residual toner and can perform cleaning simultaneously with developing. The developing devices disclosed in these patents utilize a potential difference between a developing sleeve and the photosensitive member for adhering the toner onto an exposed portion of the photosensitive member and removing residual toner adhering to an unexposed portion of the photosensitive member.
In such cleanerless image forming apparatuses, a slight amount of toner inevitably remains on the photosensitive member after the transfer, so that exposure is effected on the photosensitive member through the residual toner which has remained since preceding cycles, when the image forming cycle is repeated. Therefore, unremoved residual toner intercepts light beams at a certain portion, so that the light-intercepted portion will produce an image memory on an image formed in the next operation.
The cleanerless image forming apparatuses disclosed in the above patents employ a rotary brush charger as the charging device, so that the residual toner on the photosensitive member may be scattered by a brush into an unpatterned form.
In practice, however, only the employment of the rotary brush charger cannot sufficiently suppress generation of an image memory.